The invention relates to a high vacuum tight closure for preferably large nominal widths including a valve seat, a spreadable valve plate and means for centering and guiding the valve plate during its closing movement.
For heatable high vacuum systems, particularly for use in nuclear installations, the need for containment assemblies having metallic seals is increasing since such seals must be heatable to temperatures of more than 300.degree. C. and their radiation resistance is significantly better than that of elastomer seals.
In order to obtain the desired low leakage rate (better than 10.sup.-8 mbar.multidot.1.multidot.s.sup.-1 for a nominal width up to 1000 mm) for metallic seals, for the surfaces of the sealing members have to adapt themselves to one another by way of plastic and/or elastic deformation so that the remaining gap lies in the order of magnitude of the surface roughness. To achieve such a result, there are needed high contact pressure forces and extremely clean sealing faces that are completely free of flaws. It is therefore a disadvantageous for the sealing faces if any kind of friction is generated thereon.
German Offenlegungsschrift [Laid-Open Patent Application] No. 25 11 591, to which corresponds U.S. Pat. No. 3,973,753, discloses the provision of three beads along the outer edge of the spreadable valve plate; the outer beads are protective beads while the inner bead constitutes the actual sealing face or edge. The protective bead facing the opening to be sealed also serves to guide the plate during its closing movement. During the closing movement, this protective bead moves past the sealing face forming the valve seat so that it cannot be prevented that during the closing movement friction appears between the frontal protective bead and the sealing face. This results in undesirable grooves in the valve seat which may lead to a considerable increase in the leakage rate. Precisely in particularly large sealing systems installed in nuclear plants, faulty sealing faces result in long and costly interruptions of operation. Further the possibility is not excluded that friction appears also between the actual sealing faces before the valve plate has assumed its closed position.